Five of the six projects of the proposed Program involve the study of the consequences of mutations introduced in mice by transgenic or gene targeting technology and acting alone or in combination. These studies require expert and efficient pathological analyses of large numbers of specimens (tumors, tissues and, on occasion, embryos) to investigate tumorigenesis and/or phenotypic manifestations affecting murine development. The diagnostic procedures that are central to the implementation of the experimental plans and the progress of the Program include macroscopic anatomic examination, microscopic histology, and immunohistochemistry. The aim of this Core facility is to provide murine pathology service by an experienced pathologist combining uniquely in his extensive background expertise in both human clinical pathology and mouse development and pathology, including oncogenesis. In addition, an experienced cytogeneticist will provide services for expert analysis of tumor-related genomic instability. The Core Leader and Co-Leader initiated the function of a Mouse Pathology facility three years ago under the auspices of previous support by the NCI. Accordingly, their experience provides assurance for optimal operation of this Core. In fact, quality control is completely secured, as the procedures that are involved have already been optimized, standardized and streamlined. It is projected, therefore, that interpretations will be provided rapidly and efficiently. Moreover, the previous experience guarantees active interactions with the Program participants enhancing the development of projects involving mouse models.